A prism compressor is an optical device used to shorten the duration of a positively chirped ultrashort laser pulse by giving different wavelength components a different time delay. It typically consists of two prisms and a mirror. Figure 1 shows the construction of such a compressor. Although the dispersion of the prism material causes different wavelength components to travel along different paths, the compressor is built such that all wavelength components leave the compressor at different times, but in the same direction. If the different wavelength components of a laser pulse were already separated in time, the prism compressor can make them overlap with each other, thus causing a shorter pulse. Prism compressors are typically used to compensate for dispersion inside Ti:sapphire modelocked lasers. Each time the laser pulse inside travels through the optical components inside the laser cavity, it becomes stretched. A prism compressor inside the cavity can be designed such that it exactly compensates this intra-cavity dispersion. It can also be used to compensate for dispersion of ultrashort pulses outside laser cavities. Prismatic pulse compression was first introduced, using a single prism, in 1983 by Dietel et al. and a four-prism pulse compressor was demonstrated in 1984 by Fork et al. Additional experimental developments include a prism-pair pulse compressor and a six-prism pulse compressor for semiconductor lasers. The multiple-prism dispersion theory, for pulse compression, was introduced in 1982 by Duarte and Piper, extended to second derivatives in 1987, and further extended to higher order phase derivatives in 2009. An additional compressor, using a large prism with lateral reflectors to enable a multi-pass arrangement at the prism, was introduced in 2006. Principle of operation Almost all optical materials that are transparent for visible light have a normal, or positive, dispersion: the refractive index decreases with increasing wavelength. This means that longer wavelengths travel faster through these materials. The same is true for the prisms in a prism compressor. However, the positive dispersion of the prisms is offset by the extra distance that the longer wavelength components have to travel through the second prism. This is a rather delicate balance, since the shorter wavelengths travel a larger distance through air. However, with a careful choice of the geometry, it is possible to create a negative dispersion that can compensate positive dispersion from other optical components. This is shown in Figure 3. By shifting prism P2 up and down, the dispersion